Radiant energy – Invisible radiant energy responsive electric signalling – Infrared responsive
Reexamination Certificate
1998-12-04
2001-04-17
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Infrared responsive
C250S338100
Reexamination Certificate
active
06218666
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method of determining the concentration of a gas to be analysed contained in a gas mixture by optical absorption spectroscopy, and an analyser for implementing such a method.
BACKGROUND OF THE INVENTION
In this type of method, the concentration of the gas is determined from its optical absorption characteristics by measuring the intensity of radiation in various wavelength bands transmitted through the gas mixture.
This technique is very sensitive to temperature and gives variable results when the ambient temperature changes, on the one hand because of the resulting modification of the characteristics of the components making up the analyser, in particular the semiconductor-based optical components, and on the other hand because of the variation in the absorption spectrum of the gas to be analysed as a function of temperature, this variation being due in particular to a variation in its density.
In order to compensate for the measurement errors caused by the variation in the sensitivity of the optical components as a function of temperature, it is known to introduce, into the analyser, a thermistor, the temperature coefficient of which is chosen so as to be equal to that of the photoreceiver optical component, associated with an adjusting resistor in order to set the information provided by the analyser to zero in the absence of the gas to be analysed. Thus, the influence of temperature on the photoreceiver is neutralized.
Moreover, in order to neutralize the influence of temperature on the absorption spectrum, a diode is used whose temperature dependence is similar to that of the absorption spectrum of the gas to be analysed. This diode controls a calibrating voltage supplied to the radiation source so as to compensate for the variation in gas density.
This technique requires a prior step of calibrating the analyser, during which an optical analysis cell is filled with a gas at a known concentration N and the calibrating signal is adjusted so that the information provided corresponds to this concentration value.
This technique has a number of drawbacks, especially because the calibration operation must be carried out again when the ambient temperature changes.
Furthermore, these operations are not always able to be accomplished in that it is not possible to determine, using an uncalibrated device, the absence or presence of the gas to be analysed in the environment.
Moreover, the compensating elements, such as the thermistors or diodes, only allow an approximate compensation to be made for the temperature dependence of the sensitivity of the photoreceiver and of the absorption spectrum of the gas to be analysed.
Finally, this type of method does not make it possible to compensate for the variation in the characteristics of the components other than the photoreceiver.
The object of the invention is to overcome the aforementioned drawbacks.
SUMMARY OF THE INVENTION
The subject of the invention is therefore a method of determining the concentration of a gas to be analysed in a gas mixture, by measuring at least one value of the intensity of at least one radiation transmitted through the mixture, characterized in that it comprises the steps consisting in:
calculating, from at least one measured value of the intensity of at least one radiation transmitted at ambient temperature, the value of a characteristic of the gas mixture having predetermined laws of variation as a function of temperature and as a function of the concentration of gas to be analysed;
determining the value of the said characteristic at a predefined reference temperature different from the ambient temperature from the law of variation of the characteristic as a function of temperature; and
determining the value of the concentration of the gas from the law of variation of the said characteristic as a function of the concentration.
The method according to the invention may furthermore comprise one or more of the following characteristics:
the law of variation of the said characteristic as a function of the concentration is determined during a prior calibration step by determining a set of values of the characteristic for various gas concentrations and at the reference temperature;
the characteristic consists of the transmission coefficient T(N,t) of the gas mixture, defined by the equation:
T
(
N
,
t
)
=
U
(
N
,
t
)
U
(
0
,
t
)
(
1
)
in which:
T(N,t) denotes the transmission coefficient at the temperature t of the gas mixture containing a concentration N of the gas to be analysed; and
U(N,t) and U(0,t) denote the intensity of an electrical signal formed from the output of the means for detecting the radiation transmitted through the gas mixture, at the temperature t, the mixture containing a concentration N and a zero concentration of the gas to be analysed, respectively, and the intensity of the electrical signal U(0,t) formed from a mixture containing a zero concentration of gas to be analysed being obtained during the prior calibration step;
during the prior calibration step, the intensity of the electrical signal is measured at the reference temperature for various values of the concentration of the gas to be analysed and in the absence of the gas to be analysed in the gas mixture and, for each concentration value, the transmission coefficient is calculated;
the value of the transmission coefficients at the reference temperature is determined during the analysis of the gas mixture from the following equation:
T
(
N
,
t
ref
)
=
T
(
N
,
t
a
)
ln
T
(
N
0
,
t
ref
)
ln
T
(
N
0
,
t
a
)
(
2
)
or from the equation:
T
(
N
,
t
ref
)
=
1
-
(
1
-
T
(
N
,
t
a
)
)
1
-
T
(
N
0
,
t
ref
)
1
-
T
(
N
0
,
t
a
)
(
3
)
in which:
T(N,t
ref
) and T(N,t
a
) denote the transmission coefficient of the gas mixture for a concentration N of the gas to be analysed at the reference temperature and at the ambient temperature, respectively; and
T(N
0
,t
ref
) and T(N
0
,t
a
) denote the value of the transmission coefficient of the gas mixture for a pre-determined concentration N
0
of the gas to be analysed at the reference temperature and at the ambient temperature, respectively;
during the prior calibration step, the intensity of the electrical signal is measured for the said gas concentration N
0
and for various temperature values encompassing the reference temperature and the ambient temperature, and, for each temperature value, the transmission coefficient is calculated.
The subject of the invention is also an analyser for determining the concentration of a gas in a gas mixture contained in an optical analysis cell, comprising means for emitting a radiation through the optical cell and means for detecting the radiation transmitted, both these means being connected to means for calculating the concentration of the gas, characterized in that the means for calculating the concentration of the gas comprise means for calculating the transmission coefficient of the gas mixture, at ambient temperature, means for calculating the corresponding value of the transmission coefficient at a reference temperature different from the ambient temperature and means for calculating the concentration of the gas by comparing the calculated value of the transmission coefficient at the reference temperature with a set of predetermined values stored in a memory, each value corresponding to a concentration value of the gas.
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patent: 4579456 (1986-04-01), Gramont
patent: 5281817 (1994-01-01), Yelderman et al.
patent: 5572031 (1996-11-01), Cooper et al.
patent: 5585635 (1996-12-01), Graham
patent: 31 16344 A1 (1982-11-01), None
patent: 0 084 726 A1 (1983-08-01), None
patent: 0 318 752 (1989-06-01), None
patent: 58-135940 (1983-08-01), None
patent: 61-097552 (1986-05-01), None
patent: 6-222003 (1994-08-01), None
patent: 6-281477 (1994-10-01), None
Francuzova Svetlana Igorevna
Gamartz Jemeljan Mihailovic
Krilov Vladimir Arkadevic
Lukica Ivan Gavrilovic
Gabor Otilia
Hannaher Constantine
Oldham France S.A.
Young & Thompson
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